[R-sig-eco] Transforming logits to probabilities

[Manuel Spínola]

Dear list members,

I am using function invlogit from the "arm" package to get the 
difference in probabilities for a explanatory factor from the model 
below (I am working with factor "iluminacion").

 > invlogit(-0.4241 - 0.4804 * 0) - invlogit(-0.4241 - 0.4804 * 1) # 
difference in probability for iluminacion
[1] 0.1074094

How can I transform the standard error of the difference from logit to 
probability?

The standard error in logits is 0.2416 (taken from the model below).

Best,

Manuel


 > summary(mod6)

Call:
glm(formula = condicion ~ iluminacion + animales + cielo.raso +
     piso + paredes, family = binomial, data = reglog)

Deviance Residuals:
     Min       1Q   Median       3Q      Max
-1.2527  -1.0034  -0.8276   1.2208   1.6981

Coefficients:
                  Estimate Std. Error z value Pr(>|z|)
(Intercept)       -0.4241     0.2833  -1.497   0.1343
iluminacion[T.1]  -0.4804     0.2416  -1.988   0.0468 *
animales[T.1]      0.4613     0.2385   1.934   0.0531 .
cielo.raso[T.1]   -0.1386     0.2840  -0.488   0.6254
piso[T.1]          0.1380     0.2934   0.470   0.6382
paredes[T.1]      -0.1289     0.3012  -0.428   0.6687
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

(Dispersion parameter for binomial family taken to be 1)

     Null deviance: 433.04  on 319  degrees of freedom
Residual deviance: 424.30  on 314  degrees of freedom
   (25 observations deleted due to missingness)
AIC: 436.3

Number of Fisher Scoring iterations: 4


-- 
Manuel Spínola, Ph.D.
Instituto Internacional en Conservación y Manejo de Vida Silvestre
Universidad Nacional
Apartado 1350-3000
Heredia
COSTA RICA
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mspinola10 at gmail.com
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